Document Alpha features

Today, we have alpha features that are enabled by setting
`enable-api-fields` to `"alpha"`. We only have a subset of the alpha
features listed in the documentation.

In this change, we document all the alpha features with references to
their TEPs and Releases. If applicable, we add the individual feature
flag as well.

docs/install.md

@@ -363,8 +363,8 @@ The default is `false`. For more information, see the [associated issue](https:/
 use of custom tasks in pipelines.
 
 - `enable-api-fields`: set this flag to "stable" to allow only the
-most stable features to be used. Set it to "alpha" to allow alpha
-features to be used.
+most stable features to be used. Set it to "alpha" to allow [alpha
+features](#alpha-features) to be used.
 
 - `scope-when-expressions-to-task`: set this flag to "true" to scope `when` expressions to guard a `Task` only. Set it
   to "false" to guard a `Task` and its dependent `Tasks`. It defaults to "false". For more information, see [guarding
@@ -391,10 +391,17 @@ the `feature-flags` ConfigMap alongside your Tekton Pipelines deployment.
 
 Features currently in "alpha" are:
 
-- [Tekton Bundles](./taskruns.md#tekton-bundles)
-- [Custom Tasks](./runs.md)
-- [Isolated Step & Sidecar Workspaces](./workspaces.md#isolated-workspaces)
-- [Hermetic Execution Mode](./hermetic.md)
+| Feature                                                                         | TEP                                                                                                         | Release                                                              | Individual Flag             |
+|:------------------------------------------------------------------------------- |:----------------------------------------------------------------------------------------------------------- |:-------------------------------------------------------------------- |:--------------------------- |
+| [Bundles ](./pipelineruns.md#tekton-bundles)                                    | [TEP-0005](https://github.com/tektoncd/community/blob/main/teps/0005-tekton-oci-bundles.md)                 | [v0.18.0](https://github.com/tektoncd/pipeline/releases/tag/v0.18.0) | `enable-tekton-oci-bundles` |
+| [`Runs` and `Custom Tasks`](./runs.md)                                          | [TEP-0002](https://github.com/tektoncd/community/blob/main/teps/0002-custom-tasks.md)                       | [v0.19.0](https://github.com/tektoncd/pipeline/releases/tag/v0.19.0) | `enable-custom-tasks`       |
+| [Isolated `Step` & `Sidecar` `Workspaces`](./workspaces.md#isolated-workspaces) | [TEP-0029](https://github.com/tektoncd/community/blob/main/teps/0029-step-workspaces.md)                    | [v0.24.0](https://github.com/tektoncd/pipeline/releases/tag/v0.24.0) |                             |
+| [Hermetic Execution Mode](./hermetic.md)                                        | [TEP-0025](https://github.com/tektoncd/community/blob/main/teps/0025-hermekton.md)                          | [v0.25.0](https://github.com/tektoncd/pipeline/releases/tag/v0.25.0) |                             |
+| [Graceful Termination](./pipelineruns.md#gracefully-cancelling-a-pipelinerun)   | [TEP-0058](https://github.com/tektoncd/community/blob/main/teps/0058-graceful-pipeline-run-termination.md)  | [v0.25.0](https://github.com/tektoncd/pipeline/releases/tag/v0.25.0) |                             |
+| [`PipelineRun` Timeouts](./pipelineruns.md#configuring-a-failure-timeout)       | [TEP-0046](https://github.com/tektoncd/community/blob/main/teps/0046-finallytask-execution-post-timeout.md) | [v0.25.0](https://github.com/tektoncd/pipeline/releases/tag/v0.25.0) |                             |
+| [Specifying `onError` for a `Step`](./tasks.md#specifying-onerror-for-a-step)   | [TEP-0040](https://github.com/tektoncd/community/blob/main/teps/0040-ignore-step-errors.md)                 | [v0.27.0](https://github.com/tektoncd/pipeline/releases/tag/v0.27.0) |                             |
+| [Implicit `Parameters`](./taskruns.md#implicit-parameters)                      | [TEP-0023](https://github.com/tektoncd/community/blob/main/teps/0023-implicit-mapping.md)                   | [v0.28.0](https://github.com/tektoncd/pipeline/releases/tag/v0.28.0) |                             |
+| [Windows Scrips](./tasks.md#windows-scripts)                                    | [TEP-0057](https://github.com/tektoncd/community/blob/main/teps/0057-windows-support.md)                    | [v0.28.0](https://github.com/tektoncd/pipeline/releases/tag/v0.28.0) |                             |
 
 ## Configuring High Availability